Nanosatellite solar cell regulator

ABSTRACT

A solar cell regulator in a nanosatellite includes a pulse width modulated DC-DC boost converter and a peak power tracking controller for converting solar cell power to bus power for charging of system batteries and powering loads while the controller controls the pulse width modulation operation of the converter for sensing solar cell currents and voltages along a power characteristic curve of the solar cell for peak power tracking, for determining any power data point, including a peak power point, an open circuit voltage point, and a short circuit current point along the power characteristic curve of the solar cell, and for communicating the power data to a satellite processor for monitoring the performance of the solar cell during operational use of the satellite.

STATEMENT OF GOVERNMENT INTEREST

The invention was made with Government support under contract No.FA8802-04-C-0001 by the Department of the Air Force. The Government hascertain rights in the invention.

REFERENCE TO RELATED APPLICATION

The present application is related to U.S. Pat. No. 6,127,621, issuedOct. 2, 2000, entitled “Power Sphere” and U.S. Pat. No. 6,396,167, B1,issued May 28, 2002, entitled “Power Distribution System”.

FIELD OF THE INVENTION

The invention relates to the field of power systems for nanosatellitesand picosatelites. More particularly, the present invention relates tosolar cell regulators in solar array power distribution systems forsmall satellites.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,127,621 and U.S. Pat. No. 6,396,167-B1, issued toSimburger, teach a distributed power system where a power ring bus isused to connect multiple DC-DC converters in parallel with each DC-DCconverter being connected to a solar cell or battery. Each DC-DCconverter is supplied with an individual control regulator and suppliedcurrent to the bus based upon measurements of bus voltage of the powerring bus. Solar cells are body mounted on the various sides of thepicosatellite. The power ring bus architecture solves the problem ofobtaining the maximum amount of electric power from a solar array. Thesolar cell array has multiple panels that are arranged on a surface ofthe picosatellite. The parallel-connected regulators each include aboost converter, a pulse width modulator (PWM), and a voltage comparatorcircuit for performing the control function of regulating the amount ofpower to be delivered onto the ring power bus from a solar cell. Thedistributed power system has a loading problem. As the load increases onthe power ring bus, the PWM increases the current output from the solarcell beyond the maximum power point for the solar cell thereby reducingthe amount of power delivered to the power ring bus. Without peak powertracking, the regulator control circuit and implementation algorithmcauses the output of the regulators to rapidly go to zero.

Another problem with the prior picosatellite distributed power system isan inability of the satellite to ascertain the operation efficiency ofthe solar cells, which may extend over several years of operation. Forexample, in a solar storm, solar cells can be damaged by radiationleaving the amount of power generation undetermined in the presence offixed mission power requirements. Power systems on current satellite useshunt regulators or an unregulated bus to which the solar array isconnected. Using an unregulated bus with shunt regulators, it isimpossible to obtain the voltage current characteristic from the actualcells on the solar array. As such, the health of the solar array cannotbe determined by a central processing system. With a centralized powersystem, there is usually a single regulator circuit, which controls allof the individual solar cell strings, or in the case of an unregulatedbus, all of the solar cells are continuously connected to the bus. Thus,to measure the current and voltage characteristics of the actual solararray, the whole array must be driven from open circuit conditions toshort circuit conditions. This is not possible for conventional solararrays with conventional control architectures. These and otherdisadvantages are solved or reduced using the invention.

SUMMARY OF THE INVENTION

An object of the invention is to provide a solar cell regulator for peakpower tracking of solar cell power.

Another object of the invention is to provide a solar cell regulator forpeak power determination of a solar cell.

Yet another object of the invention is to provide a solar cell regulatorfor open voltage determination of a solar cell.

Still another object of the invention is to provide a solar cellregulator for short circuit current determination of a solar cell.

A further object of the invention is to provide a solar cell regulatorfor peak power, short-circuit current, and open voltage determination.

Yet a further object of the invention is to provide a solar cellregulator for providing power data communicated to a central processorof a satellite.

Yet a further object of the invention is to provide a solar cellregulator for providing power data to a central processor of a satelliteupon request from the central processor.

A solar cell regulator includes a converter for converting solar cellpower to bus power and includes a controller for controlling theconverter for peak power tracking of the solar cell and for generatingsolar cell power data. The solar cell power data includes a peak powerpoint, open circuit voltage, and a short circuit current, collectivelyas power data indicating the health of the solar cell. In the preferredform, the controller can communicate the power data over a ring data busto a satellite processor for ascertaining the operational health of thesolar cell while connected through the regulator to the bus. Thesatellite processor may further issue periodic requests for power datato the controller for collecting power data upon demand. These and otheradvantages will become more apparent from the following detaileddescription of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a nanosatellite power system.

FIG. 2 is a block diagram of a solar cell regulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention is described with reference to thefigures using reference designations as shown in the figures. Referringto the FIG. 1, a nanosatellite power system includes a ring power busfor routing power and preferably includes a ring data bus forcommunicating data. A load draws power from the ring power bus. Abattery charger and regulator is connected between the ring power busand battery for charging and discharging battery power from and onto thering power bus as needed. First and second solar cell regulators areconnected to the ring power bus for coupling solar power onto the ringpower bus. First and second pairs of solar cells are respectivelyconnected to first and second pairs of blocking diodes that are in turnrespectively connected to the first and second regulators. The solarcells and the blocking diodes are shown in pairs as is common in solararrays disposed on orbiting satellites. The nanosatellite power systemcan include any number of solar cells and parallel strings of solarcells in the solar cell array.

Referring to FIGS. 1 and 2, and more particularly to FIG. 2, a solarcell regulator includes a peak power controller and a DC-DC boostconverter. The boost converter is coupled to the ring power bus forcoupling solar power from the solar cells onto the ring power bus. Anisolation diode in the regulator isolates bus power from the converter,as is common practice so that output current Iout from the regulatorflows towards the ring power bus. An input current sensor measures Iinfrom the solar cells. The controller senses the input current Iin. Theinput current Iin has an associated input voltage Vin. The controllersenses the input voltage Vout. An output current sensor measures theoutput current lout that is sensed by the controller. The output currentlout is conducted to the ring bus having an associated output busvoltage VBus that is also sensed by the controller. The converter candrive the VBus ring bus voltage of the ring power bus through anisolation diode. The isolation diode provides isolation from the bus inthe event of a catastrophic failure in an individual regulator. Thus,the output of an individual regulator can be driven to zero voltswithout causing the voltage on the ring power bus to drop to zero.Parallel-connected regulators and battery regulators can be used tomaintain bus voltage and continue to supply power to the load.

The converter compares a reference voltage VREF to the bus voltage VBusfor generating an error signal used to pulse width modulate (PWM) apulse width modulation signal within the converter for controlling theamount of power coupled to the bus. The input current sensor sensesinput current Iin from the solar cells also providing an input voltageVin to the regulator. During normal operation, the pulse widthmodulation is varied so that input power (Vin)(Iin) equals the outputpower (VBus)(IOut). The peak power controller senses the input currentIin and the input voltage Vin. The peak power tracker then provides acontrol signal to the converter. The control signal is for controllingthe pulse width modulation and, hence, the input power and output powerso as to track maximum peak power generation of the solar cells. Thepeak power controller performs peak power tracking. The peak powertracking is used to obtain power data accurately indicating the state ofthe health of operation of the solar cells or the health of an array ofcells deployed satellite on orbit.

The peak power controller generates and adjusts the control signal tovary the duty cycle of the pulse width modulated signal so as to varythe input voltage Vin and input current Iin over an operational rangefor determining the peak power operating point. The controller can thencontrol the converter to operate at the peak power point so that peakpower from the solar cells is delivered onto the ring power bus. Byvarying the duty cycle, the controller can also sense Vin whilecontrolling the converter to put the solar cells into a short circuitcondition for determining the short circuit current of the solar cells.By varying the duty cycle, the controller can also sense Iin whilecontrolling the converter to put the solar cells into an open-circuitcondition for determining the open circuit voltage of the solar cells.The peak power point, short circuit current, and open circuit voltageare power data indicating the health of the solar cells. The controllercan then communicate the power data to the nanosatellite processor. Thecontroller could as well function as a slave device to the processor bygenerating and communicating the power data to the processor in responseto requests from the processor to the solar cell regulator. In thismanner, the processor can determine the health of the solar cells formanagement of power resources during operational use.

The regulator has been improved to add more capability for powermanagement. The peak power controller controls the DC-DC boost converterand implements a peak power-tracking algorithm. Two different preferredpeak-power-tracking algorithms may be used among many well-knownpeak-power-tracking algorithms. The first preferred peak-power-trackingalgorithm periodically determines the peak power voltage of the solarcells. Once the peak power voltage is determined, and then thepeak-power-tracking algorithm compares the input voltage from the solararray with a value of a predetermined peak power voltage. This peakpower voltage value can be supplied by the processor and communicated tothe controller during operational use. When the input voltage Vin dropsbelow the predetermined peak power voltage, then the peak-power trackercontrols the DC-DC boost converter to modulate the PWM signal to reducecurrent demand on the solar cell. This peak power function prevents thePWM signal from increasing solar cell current beyond the peak poweroperating point of the solar cell. The peak power tracker provides fororderly increases or decreases of input power from the solar cells thatmay be due to changes in the intensity of the sunlight.

The second preferred peak-power-tracking algorithm is used to measurethe input voltage Vin and input current Iin being supplied by the solarcells. These Vin and Iin values and the power product (Vin)(Iin) valuescan be stored in a memory, not shown, in the controller. This secondpeak-power-tracking algorithm is repeated continuously with the measuredVin and Iin values for at least two consecutive iterations stored in thememory at all times. The most recent Vin and Iin sensed measurements arecompared with the prior measurements to determine when an increase incurrent demand by the DC-DC converter results in a decrease in powerbeing supplied by the solar cells. When an increase in current demand bythe DC-DC converter results in a decrease in power being supplied by thesolar cells, then the peak-power controller sends a control signal tothe DC-DC converter to control the PWM signal to reduce current demandon the solar cell. The peak power tracking function prevents the PWMsignal from increasing solar cell current beyond the peak poweroperating point of the solar cell. The peak power controller providesfor orderly increase or decrease of power output from a solar cell dueto changes in the intensity of the sunlight.

For operation in the sunlight portion of the orbit, a first level ofcontrol is the PWM of the DC-DC boost converter, which provides aregulated bus with regulated voltages typically between 9.5 and 10.5volts. The PWM DC-DC converter could increase the current demand on thesolar array beyond the peak power point. When the current demands exceedthe peak power point, the power output of the converter would collapseto zero. To prevent a power collapse to zero from happening, thecontroller also monitors the bus voltage VBus and output current IOutand invokes the peak power-tracking algorithm. Thus, when an increase incurrent demand from the PWM DC-DC converter results in a decrease inpower output, the controller commands converter to draw less currentfrom the solar cells. The controller also monitors the solar arrayvoltage Vin and turns the PWM DC-DC converter off when the solar arrayvoltage drops below a predetermined voltage, such as 3.0 volts. Thecontroller turns the converter back on through PWM control when thevoltage exceeds 3.2 volts.

An additional benefit of the regulator is the ability to directlymeasure the Iin current and Vin voltage characteristic of the individualsolar cells or string of solar cells being controlled by the regulator.The voltage current characteristic is obtained by having the controllerprovide the control signal to the converter for controlling the dutycycle of the PWM signal, which causes the PWM signal to demand zerocurrent from the solar cell. Next, the controller measures the inputvoltage from the solar cell and verifies that zero current is beingdelivered to the DC-DC converter. Next, the controller controls the PWMsignal to increase the current from the solar cell from zero to amaximum in small increments. At each current increment, the controllerrecords the Vin voltage and Iin current output form the solar cells. Bycontinuing to record power points from zero to the maximum currentdemand, the current and voltage characteristic of the solar cells iseffectively measured. The power data can thus include the current andvoltage characteristic curve that will indicate the peak power point aswell as providing open circuit voltage and short circuit power data. Thepower data is useful in determining the health of the solar array andcan be used to determine the amount of degradation of each part of thesolar array that has been experienced over the mission lifetime of thenanosatellite. The power data can also be used to verify and updatedegradation models, which are used to predict the useful remaining lifefor the particular satellite. The communication of data on the ringpower bus enables the processor to request the controller to place thesolar cell at any one power point along the power characteristic curveof the solar cell. The communication of data on the ring power busenables the controller to communicate any one power point along thepower characteristic curve of the solar cell to the processor. Thecommunication of data on the ring power bus enables the controller tointerrupt the processor during critical power events. The communicationof data on the ring power bus and the ability to control the inputvoltage and input current for measuring points along the powercharacteristic curve enables the processor to determine the health ofthe solar cell array.

Each of the regulators preferably has a microprocessor-based peak powercontroller and the DC-DC boost converter. One or more regulators may beused for a solar array. For example, the solar array may have four solarcells connected in series to form a solar cell string. The string may beconnected to a respective solar cell regulator. Two series connectedsolar cell strings may then be connected in parallel as a dual stringarray. Each dual string array may be connected to respective solar cellregulators. Two of these dual string arrays may be respectively disposedon two opposing faces of the nanosatellite. The entire solar cell arrayof two dual string arrays may be connected to a single solar arrayregulator.

The invention is directed to a regulator preferably for solar cellsdisposed on small satellites for maximizing the power output of multiplesolar array panels, each of which may be operating at differenttemperatures and have different orientations with respect to the sun.The regulator implements a peak-power-tracking algorithm preferably in acontroller to automatically provide peak power output of the solararrays to a power bus under all conditions. The regulator preferablyincludes a controller that communicates power data to a satelliteprocessor for power management. Those skilled in the art can makeenhancements, improvements, and modifications to the invention, andthese enhancements, improvements, and modifications may nonetheless fallwithin the spirit and scope of the following claims.

1. A nanosatellite solar cell regulator comprising, a nanosatellite, aregulator for peak power tracking of a solar cell, the regulatordisposed between the solar cell and a nanosatellite ring power bus, theregulator comprising, an input current sensor for measuring an inputcurrent Iin from the solar cell, a boost converter for receiving aninput voltage Vin at the input current Iin from the solar cell providinginput power (Vin)(Iin), the regulator providing an output current Iout,the regulator converting the input power to output power (VBus)(Iout)where VBus>Vin, the output power is communicated onto the ring power busvia an isolation diode between the boost converter and a VBus measuringdevice, the ring power bus having an output bus voltage VBus, the solarcell having a peak power point, and a controller for monitoring theinput power and for controlling the boost converter for providing thepeak power output of the solar cells.
 2. The device of claim 1 wherein,the solar cell is a string of series connected solar cells.
 3. Thedevice of claim 1 wherein, the solar cell is an array of solar cells. 4.A nanosatellite solar cell regulator comprising, a nanosatellite, aregulator for peak power tracking of a solar cell, the regulatordisposed between the solar cell and a nanosatellite ring power bus, theregulator for providing power data of the solar cell to a nanosatelliteprocessor through a ring data bus, the regulator comprising, an inputcurrent sensor for measuring an input current Iin from the solar cellproviding an input voltage Vin, an output current sensor for measuringan output current Iout of the power bus having a bus voltage VBus, aboost converter for receiving an input voltage Vin and input current Iinfrom the solar cell providing input power (Vin)(Iin), the regulatorproviding an output current Iout, the regulator converting the inputpower to output power (VBus)(Iout) where VBus>Vin, the output powerconducted to the ring power bus, the output power is communicated ontothe ring power bus via an isolation diode between the boost converterand a VBus measuring device, the ring power bus having an output busvoltage VBus, the solar cell having a peak power point, the solar cellhaving an open circuit voltage, the solar cell having a short circuitcurrent, the solar cell having a power characteristic curve comprisingthe peak power point and the short circuit current and the open circuitvoltage, and a controller for monitoring the input power and forcontrolling the boost converter for providing peak power output of thesolar cell, the controller reducing the output power when the peak powerpoint is exceeded, the controller monitoring the input power fordetermining power data comprising the peak power point, the controllerfor communicating the power data to the nanosatellite processor, thepower data comprising a data point on the power characteristic curve. 5.The device of claim 4 wherein, the power data comprises the peak powerpoint and the open circuit voltage and the short circuit current.
 6. Thedevice of claim 4 wherein, the solar cell is a string of seriesconnected solar cells.
 7. The device of claim 4 wherein, the solar cellis an array of solar cells.
 8. The device of claim 4 wherein, the boostconverter compares a reference voltage to VBus for determining an errorsignal, and the boost converter modulating a pulse width modulationsignal by the error signal for converting input power into output powerat the peak power point.
 9. The device of claim 4 wherein, controllercontrols the boost converter to reduce the input current Iin to zero fordetermining the open circuit voltage, and the power data comprises theopen circuit voltage.
 10. The device of claim 4 wherein, the controllercontrols the boost converter to short circuit the input voltage Vin fordetermining the short circuit current, and the power data comprises theshort circuit current.
 11. The device of claim 4 wherein, the boostconverter compares a reference voltage to the bus voltage VBus fordetermining an error signal, the boost converter modulating a pulsewidth modulation signal by the error signal for converting the inputpower into the output power, and the controller controlling the pulsewidth modulation to a plurality of power points of the input powerbetween an open circuit and a short circuit of the power characteristiccurve.
 12. The device of claim 4 wherein, the boost converter compares areference voltage to the bus voltage VBus for determining an errorsignal, the boost converter modulating a pulse width modulation signalby the error signal for converting input power into output power, thecontroller controlling the pulse width modulation to a plurality ofpower points of the input power between an open circuit and a shortcircuit of the power characteristic curve, and the power data comprisesa plurality of data points on the power characteristic curve.
 13. Thedevice of claim 1 wherein, the nanosatellite comprises a battery, thering power bus is coupled to the battery, and the solar cells chargesthe battery through the regulator and ring power bus.
 14. The device ofclaim 1 wherein, the data bus is a ring data bus, and the power data iscommunicated onto the ring data bus of the nanosatellite, the ring databus being connected to the nanosatellite processor.
 15. The device ofclaim 4 wherein, the nanosatellite processor commands the controller toopen circuit the solar cell for determining an open circuit voltagepoint along the power characteristic curve.
 16. The device of claim 4wherein, the nanosatellite processor commands the controller to shortcircuit the solar cell for determining a short circuit current pointalong the power characteristic curve.
 17. The device of claim 1 wherein,the nanosatellite processor communicates a request for power data to thecontroller.
 18. The device of claim 1 wherein, the nanosatelliteprocessor communicates a request for power data to the controller, andthe controller communicates power data to the processor.
 19. The deviceof claim 1 wherein, the nanosatellite processor communicates a requestfor power data to the controller, and the controller communicates powerdata to the processor, the power data comprising power points along thepower characteristic curve.
 20. A method of operating an electric powersystem for a nanosatellite comprising the steps of, providing ananosatellite and a nanosatellite ring power bus electric powerdistribution system, determining a reference voltage corresponding to amaximum power point from a solar cell power source including one or moresolar cells, supplying power from the solar cell power source to a boostconverter, boosting the voltage of the supplied power in the boostconverter, supplying power from a boosted voltage power output of theboost converter to the nanosatellite ring power bus, and controlling theboost converter to reduce the power supplied by the solar cell powersource when the corresponding solar cell power source voltage fallsbelow the reference voltage.
 21. The method of claim 20 furtherincluding the steps of, short circuiting the solar cell power source byoperation of the boost converter in a short circuit mode, and preventingoverloading the ring power bus by providing an isolation diode betweenthe boost converter and the ring power bus.
 22. A method of operating anelectric power system for a nanosatellite comprising the steps of,providing a nanosatellite and a nanosatellite ring power bus electricpower distribution system, supplying power from the solar cell powersource to a boost converter, boosting the voltage of the supplied powerin the boost converter, supplying power from a boosted voltage poweroutput of the boost converter to the nanosatellite ring power bus, andcontrolling the boost converter to reduce the power supplied by thesolar cell power source when a commanded power increase results in alower power output.
 23. The method of claim 22 further including thesteps of, short circuiting the solar cell power source by operation ofthe boost converter in a short circuit mode, and preventing overloadingthe ring power bus by providing an isolation diode between the boostconverter and the ring power bus.